Tai Chi as a form of physical discipline or exercise is fine, as is learning martial arts to defend oneself or others.
However, mixing in religious ideas, such as believing we have an energy (chi) within ourselves that we can harness and control is not compatible with Catholicism.
In addition, similar to Yoga, in its religious expression Tai Chi espouses pantheism, i.e., that there is no personal God and that we therefore somehow are God. Consequently, this erroneous religious aspect must also be avoided.
In short, Tai Chi as a form of exercise is fine, because its practitioners don’t have a monopoly on certain physical movements. But Tai Chi as a means of religious promotion or practice is not morally permissible. In this light, Catholics should avoid any class in which the leader is promoting or practicing the religious aspects of Tai Chi vs. simply using the class as means of physical fitness for themselves and others.
For more on this subject, see the Church document Jesus Christ the Bearer of the Water of Life, which hits on related topics, including the practice of yoga.